Interpretive Summary: Plant carotenoids are the primary dietary source of provitamin A for humans. As one of the most widespread nutrient deficiencies, about 400 million people in developing countries currently suffer from dietary vitamin A deficiency because of a lack of carotenoids in their diets. Thus, biofortifying staple crops with carotenoids is considered to be a very effective and sustainable approach to help afflicted populations to fight against vitamin A deficiency. A comprehensive understanding of the mechanisms of carotenoid biosynthesis and accumulation is essential for the development of carotenoid-enriched staple food crops. However, our current knowledge in this area is limited to the carotenoid biosynthetic pathway. Using a unique Or mutant in cauliflower as a model system, we have identified a novel regulatory gene that controls carotenoid biosynthesis and accumulation in plants and elucidated its functional role in regulating carotenoid accumulation. With the discoveries reported herein, we are extremely hopeful that the combination of manipulating both regulatory and biosynthetic genes will be a more effective strategy to enhance carotenoid levels in staple crops to achieve the desired enrichments of carotenoids.

Technical Abstract:
Carotenoids are a group of pigments that are indispensable to plants and essential to human diets. In spite of recent significant progress in our understanding of carotenogenesis in plants, the control mechanisms that govern overall carotenoid accumulation remain largely unknown. The Orange (Or) gene mutation in cauliflower induces the accumulation of high levels of B-carotene in various tissues that are normally devoid of carotenoids, turning them orange. Using positional cloning, we have identified a single candidate gene representing Or and successfully verified its role in carotenogenesis by functional complementation in wild type cauliflower. We show that Or encodes a plastid-associated protein containing a cysteine-rich zinc finger domain found in DnaJ-like molecular chaperones, and is expressed highly in very young leaves, curds, and flower buds. Or appears to be plant-specific and is highly conserved among divergent plant species. Analyses of the gene, the gene product, and the cytological effects of Or transgene suggest that Or functions in triggering the differentiation of proplastids or other non-colored plastids into chromoplasts, providing a metabolic sink for carotenoid accumulation. Moreover, we demonstrate that Or functions across plant species to induce carotenoid accumulation in both a model plant species of Arabidopsis and a major staple food crop of potato. Our findings provide evidence that controlling the formation of chromoplasts is an important mechanism by which carotenoid accumulation can be regulated in plants.